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Topic: Any rule of thumb for costs of a wide, squat rocket? (Read 9481 times)

Im asking this question here because it applies to a pet idea i have had for intermediate versions of the ICT, where you go straight to the final tank radius etc but just make shorter stages (or at least a shorter first stage) and attach less engines, even though this is suboptimal.

However Im not asking for discussion of that specific idea here. I am just asking if someone could produce a basic rule of thumb for the costs of building wide and squat. Im guessing there is a fairly simple relationship between width and avoidable losses of efficiency, extra fuel and tankage requirements etc for the same payload.

Obviously, many designers have done the hard work that concludes tall and narrow is more cost effective. On the other hand the shuttle seems to demonstrate you can have a pretty ungainly shape and still get to orbit. I would like to get a feel for the actual numbers, and if there is a fairly simple relationship I can take away, so I have some quantitative idea of what proportion of costs you are adding or saving.

But in regards to most current rockets, their relatively thin diameter are more driven by transport and tooling/construction restrictions.

F9's thinness is due to road transport.Proton is limited to 4.1 because of trail transport.Angara cores needed to be thinner (2.9m) to handle narrow railroad turns.Delta IV barely fits into its transport ship.... and the list goes on.

Obviously, many designers have done the hard work that concludes tall and narrow is more cost effective.

Not really. Narrow has lower drag, but higher bending loads and also not optimal for weight. The most optimal shape is a sphere, the most optimal cylinder is one with h = 2r - neither of those are good aerodynamically.

I have vague memory that for rocketry, length of 7 diameters is close to optimal, but don't quote me on this.

Rockets often becoming longer than optimal because of transportation limitations and because change in factory tooling (and doors, roof height etc) is very expensive.

America has the best transportation system in the world. Roads, interstate highway system, railroads, more inland waterways than anyone else, airports, etc. However, for the widest loads, the inter-coastal waterways and larger river systems can only handle 10-12m in width. Railroads only about 3.7m but length due to turns can be a problem, but did carry the shuttle solid boosters but separated into segments for turns. The interstate highway system maximum width and length is what the Falcon 9 currently is.

I think the above is the reason SpaceX is choosing 12m for their BFR/ITS rockets and spacecraft. That is the largest diameter the current river and waterway system can handle. Transportation does have its limitations. The Saturn V was 10m, and the proposed Nova rocket of the 1960's was going to be 12m in diameter. The largest rocket ever designed on paper was Sea Dragon, and it was 20m in diameter, but was going to be built at a submarine shipyard, towed out to sea, and launched.

Hope this helps.

I have mentioned, and I think it is a good idea, for SpaceX to build a Mini-ITS system 12m in diameter, same as full ITS. Make it shorter with fewer engines to begin with. It could be launched from the Cape. Then when the time comes stretch and add engines, and build a new launch site. Falcon was to start out as Falcon 5, shorter but the same diameter as F9. They decided to go ahead and add the maximum number of Merlin engines and stretch to the limit of the interstate highway system.

America has the best transportation system in the world. Roads, interstate highway system, railroads, more inland waterways than anyone else, airports, etc. However, for the widest loads, the inter-coastal waterways and larger river systems can only handle 10-12m in width.

While mostly true, with some flexibility on plant location, you can transport some truly large loads -- e.g., most of the Mississippi River is rated for nearly 17 meters bridge clearance, the Gulf Coast inter-coastal waterway is rated for nearly 20 meters bridge clearance, and so on.

Obviously, many designers have done the hard work that concludes tall and narrow is more cost effective.

Not really. Narrow has lower drag, but higher bending loads and also not optimal for weight. The most optimal shape is a sphere, the most optimal cylinder is one with h = 2r - neither of those are good aerodynamically.

I have vague memory that for rocketry, length of 7 diameters is close to optimal, but don't quote me on this.

Rockets often becoming longer than optimal because of transportation limitations and because change in factory tooling (and doors, roof height etc) is very expensive.

Fineness ratio, the ratio of the length of a body to its maximum width. There's a quote from Von Braun somewhere that the fineness ratio for orbital rockets shouldn't exceed 10-1, but that was in an era where materials science and engineering were much more limited than they are today. Many rockets today exceed 14-1. The Falcon 9 is in excess of 19-1. There's no real "optimum" that I can find a solid source on, it really depends on the design requirements.

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Fineness ratio, the ratio of the length of a body to its maximum width. There's a quote from Von Braun somewhere that the fineness ratio for orbital rockets shouldn't exceed 10-1, but that was in an era where materials science and engineering were much more limited than they are today. Many rockets today exceed 14-1. The Falcon 9 is in excess of 19-1. There's no real "optimum" that I can find a solid source on, it really depends on the design requirements.

Fineness ratio, the ratio of the length of a body to its maximum width. There's a quote from Von Braun somewhere that the fineness ratio for orbital rockets shouldn't exceed 10-1, but that was in an era where materials science and engineering were much more limited than they are today. Many rockets today exceed 14-1. The Falcon 9 is in excess of 19-1. There's no real "optimum" that I can find a solid source on, it really depends on the design requirements.

Yeah, the F9FT is skinny as hell.

Yes but the point is that the mythical optimal fineness ratio of the past decades seems to have been more based on rocket designer 'gut feelings' rather than actual hard data.

.... Railroads only about 3.7m but length due to turns can be a problem, but did carry the shuttle solid boosters but separated into segments for turns.

Actually railroads can handle all sorts of heights. Non-dimensional loads are <11ft x 12 ft (WxH) up to 130,000 lbs or so and less than 52ft in length. Dimensional loads are <12x13 (WxH) up to 180,000lbs; length can exceed 52ft. The latter is your 3.7m.Double stacked containers in well cars have a maximum height of 20ft or just over 6m. Well cars can be as long as 76 feet.

Aerodynamic losses are greater if you're wider. So you want to go taller if you can, but for a large rocket you'll be limited by the amount of thrust that can be produced in a given diameter with sea level expansion. So for a certain average vehicle density, there's a limit to how tall a cylindrical rocket can be (at least the propellant filled portion).

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Chris Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Aerodynamic losses are greater if you're wider. So you want to go taller if you can, but for a large rocket you'll be limited by the amount of thrust that can be produced in a given diameter with sea level expansion. So for a certain average vehicle density, there's a limit to how tall a cylindrical rocket can be (at least the propellant filled portion).

That was my initial guess, but everyone else here has been saying it has more to do with the size of roman horse's rear ends :-)

Can you guess any ballpark of what the loss is? Just roughly. (The shuttle shape IMO proves you can still get something to orbit.. but presumably you pay something for it)

Even if someone could estimate the difference between a rocket with atmosphere and a rocket with no drag, I think that would give a fair indication. Doubling the width would probably be something like doubling that loss, whatever it is?

Aerodynamic losses are greater if you're wider. So you want to go taller if you can, but for a large rocket you'll be limited by the amount of thrust that can be produced in a given diameter with sea level expansion. So for a certain average vehicle density, there's a limit to how tall a cylindrical rocket can be (at least the propellant filled portion).

That was my initial guess, but everyone else here has been saying it has more to do with the size of roman horse's rear ends :-)

Can you guess any ballpark of what the loss is? Just roughly. (The shuttle shape IMO proves you can still get something to orbit.. but presumably you pay something for it)

Even if someone could estimate the difference between a rocket with atmosphere and a rocket with no drag, I think that would give a fair indication. Doubling the width would probably be something like doubling that loss, whatever it is?

Some of the assumptions in that page rope just elevation change into gravity loss, which is not a good assumption, so the gravity loss is too high. So it's possible the other figures are questionable. Also, to some extent part of the gravity loss is due to trying to avoid more aero loss by shaping the trajectory to get out of the atmosphere as early as possible. That undercounts the importance of aero losses.

Also, drag loss is 156m/s for Titan in their figures.

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Chris Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Thanks. Excellent. "So the profound result is that drag loss as delta vee is directly proportional to frontal area per liftoff mass," .. so no really confusing factors need to be assumed it seems.

One sort of obvious (in hindsight) take away: A really fat 12 meter rocket that is only as tall as an F9 would not have worse performance (payload/launch mass) than a bunch of F9 all mooshed together, or launched separately for that matter. Maybe you can outperform an F9 by building taller, but we are not discussing doing worse.

There is certainly a trade between reducing aero losses by lengthening your cylinder, and increased dry mass needed to handle bending loads, and narrowing the launch commit parameters to deal with increased bending loads, and simply cost of retooling/transporting a rocket of larger diameter and how large/many engines can fit on the rocket, etc etc.

It's a long and complex trade that will likely be dominated by cost considerations (which includes transportation)..

« Last Edit: 05/27/2017 05:37 am by S.Paulissen »

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"An expert is a person who has found out by his own painful experience all the mistakes that one can make in a very narrow field." -Niels BohrPoster previously known as Exclavion going by his real name now.

Please no more intermediate ITS speculation until there's some actual evidence to support its existence. People are using each others speculation to bolster their own speculation, and it's turning into a big morass of alternative-fact for a lot of frequent posters around here. No. We're better than this.

As for width, the shape of the nose and where the taper begins and ends is just as portentous as the width of the nose.

Please no more intermediate ITS speculation until there's some actual evidence to support its existence. People are using each others speculation to bolster their own speculation, and it's turning into a big morass of alternative-fact for a lot of frequent posters around here. No. We're better than this.

As for width, the shape of the nose and where the taper begins and ends is just as portentous as the width of the nose.

It feels like you didn't read the OP very closely or are being very unclear who you are specifically responding to.

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"An expert is a person who has found out by his own painful experience all the mistakes that one can make in a very narrow field." -Niels BohrPoster previously known as Exclavion going by his real name now.

Chris Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0